The present invention relates to the field of filtering and in particular discloses an optical filtering method and apparatus.
Photonic processors for microwave signal processing functions are attractive because of their very high time-bandwidth product capabilities. Such processors can remove the bottlenecks caused by limited sampling speeds in conventional electrical signal processors. In addition, they have excellent isolation, immunity to electromagnetic interference (EMI), and remove the limitations of Optical/Electrical and Electrical/Optical conversions for processing high speed signals that are already in the optical domain. Photonic processors also have the ability of attaining extremely high resolution and microwave frequencies.
Recently, there has been considerable interest in photonic processing for microwave filtering applications, and a variety of structures have been reported. A common objective is to increase the Q and frequency selectivity of these filters. This is more difficult to realise for bandpass filters, because it requires an increase in the number of taps in the discrete time signal processor. For this reason, passive structures for photonic processors give limited Q values. Active structures can achieve much higher Q values, but are limited by the requirement that they operate close to the lasing threshold. Also, the fundamental frequency and finesse of these filters is limited because of the minimum length of erbium fibre that can be used. Previously, the present inventors have reported an active photonic signal processor that exhibits a Q of 325, however it is difficult to increase the Q further in this structure because of the onset of lasing.
In accordance with a first aspect of the present invention, there is provided an active-passive signal bandpass filter comprising:
an active filter an active filter arranged in use to operate at a fundamental frequency which is a sub-multiple of a desired filter frequency of the bandpass filter; and
a passive filter arranged in use to eliminate any pass bands in the frequency response of the active filter other than at the desired filter frequency for providing the pass band signal of the bandpass filter.
The active filter can comprise an infinite impulse response filter and the passive filter can comprise a finite impulse response filter.
The active and passive filters can operate on photonic signals and an input signal to the bandpass filter can comprise an optical input signal. The filter preferably operates at microwave frequencies and the input signal can be created via the modulation of the optical signal by a microwave frequency optical oscillator.
The passive filter can comprise a plurality of passive filter elements each comprising a notch filter which, in combination, have high attenuation characteristics for frequencies outside the desired filter frequency and low attenuation of the desired filter frequency. The number of passive filter elements can, for example, be 3.
The passive filter may be formed from optical fibre components.
Alternatively, the passive filter may be formed from optical planar integrated circuits.
A post filter element can be further interconnected to the passive filter, the post filter element providing for further rejection of non desired filter frequencies.
The active filter may comprise an active delay line filter.
The active delay line filter can comprise an optical fibre which comprises two gratings that define a pass length between them, the optical fibre being erbium doped and the active delay filter further comprising a pump laser for providing the gain of the active delay line filter.
Alternatively, the active filter may comprise an optical planar integrated circuit.
The optical planar integrated circuit may comprise optical waveguides for providing the delay function of the active delay line filter and doped optical waveguides for providing the gain function of the active delay line filter; and waveguide gratings in the optical waveguides defining a pass length therebetween.
The active filter may be a tunable active filter for wavelength tuning the filter frequency of the filter.
In one embodiment, the tunable active filter may comprise chirped gratings defining a plurality of path lengths, each path length being associated with a predetermined wavelength of a laser pumping the active delay line filter.
This can enable the filter frequency to be changed.
The present invention may alternatively be defined as providing a photonic processor having a quality factor in excess of about 325.